a. Field of the Invention
This invention refers to the field of radio-frequency (RF) driven arc lamps in which the structure includes a closed waveguide, and particularly to those lamps which utilize a magnetron as the source of power.
b. Description of the Prior Art
These lamps employ an ionizable medium enclosed in a sealed transparent envelope which produces visible light or ultraviolet light when excited by an intense microwave field. The lamp envelope or bulb is enclosed in a metal container or cavity which confines the microwaves while providing for the escape of the light, usually by means of a metal screen. Microwaves are admitted into the cavity through an aperture which connects to the adjoining waveguide, the other end of which couples to the magnetron.
RF power from the magnetron travels through the waveguide to the cavity and excites the discharge lamp. Any power that is not absorbed by the lamp reflects back to the magnetron. The aperture defining the end of the cavity may be used to define a resonance in the cavity which intensifies the fields at the bulb to provide increased power absorption, thus reducing the reflected power.
A magnetron is a self-excited oscillator with a direct connection between its resonator and the output load. Any reflection from the load has a strong effect on the performance, changing the operating frequency, the power output and the operating stability. Strong reflections at a particular phase known as the "sink" reduce the stored energy in the magnetron's resonator, causing instability and frequency jumping.
The lamp itself places several different requirements on its power source. Before ionization, gases in the bulb do not absorb microwave power. The electric field intensity within the bulb must be built up to a high level to achieve breakdown. Once ionization occurs, the bulb must heat to evaporate any condensed fill materials. The impedance of the bulb is much lower than the non-ionized case, and changes as the bulb heats, bringing the condensates into the discharge. And finally the long term operating condition is reached in which light output efficiency is the dominant concern.
These impedance changes result in a variety of reflected values at the magnetron. The designer can adjust the aperture of the cavity, the length of the waveguide and may add a variety of tuning elements into the waveguide. The goal is to keep the high reflection before ionization away from the sink, to avoid frequency-jumping during the warm-up cycle and to provide a good match with stable characteristics during long-term operation.
Other considerations may also enter into the design. The product needs to be economical, compact in size, durable, and reproducible. Cost prevents the use of isolators. Compact size holds the waveguide to a minimum length.
While many types of irises and posts are well-known in microwave design, the tuning element frequently used in microwave arc lamps is the capacitive screw or a fixed height knob of the same size. This has the advantage of attaching to only one wall and is more easily installed than a post which must contact two opposite walls. When a waveguide length (between the magnetron antenna and the coupling slot) greater than half a guide-wavelength is available, this capacitive tuner may be used to match a moderate mismatch of any phase. The tuner has two effects. The reflection coefficient is added to the reflection coefficient of the load beyond it. Secondly, the effective length of the waveguide is increased by a small amount.